Process for the multi-reactor synthesis of zeolite crystals having a controlled particle size

ABSTRACT

The present invention relates to a process for preparing zeolite crystals having a multimodal particle size distribution, and the sizes of which are between 0.02 μm and 20 μm, said process comprising feeding at least two reactors each with a synthesis gel capable of forming zeolite crystals, carrying out a crystallization reaction, in parallel, in each of the at least two reactors, and mixing the reaction media of the at least two reactors, after the start of at least one of the crystallization reactions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the national phase of International Application No.PCT/FR2018/050571, filed 12 Mar. 2018, which claims priority to FrenchApplication No. 1752199, filed 17 Mar. 2017. The disclosure of each ofthese applications is incorporated herein by reference in its entiretyfor all purposes.

FIELD

The present invention relates to the field of zeolites, more preciselythe field of the industrial synthesis of zeolite crystals and moreparticularly that of the industrial synthesis of zeolite crystals havinga controlled particle size.

BACKGROUND

The synthesis of zeolite crystals (or more simply “zeolite synthesis” inthe remainder of the present document) is carried out conventionally inindustry in a large-sized stirred “batch” reactor, generally withheating of the synthesis gel and/or reaction medium by injection ofsteam and/or by a heating jacket. The preparation of the synthesis gelconsists in mixing a sodium aluminate solution with a sodium silicatesolution, it being possible for this mixing to be carried out either inequipment upstream of the crystallization reactor or directly in thecrystallization reactor.

It is then often necessary to carry out a low-temperature maturingphase, generally at a temperature below 40° C., for a longer or shorterduration, generally varying from a few tens of minutes to a few tens ofhours, depending on the type of zeolite structure desired. This maturingphase makes it possible to form seed crystals that will give, by thegrowth thereof, zeolite crystals after the crystallization phase carriedout at higher temperature.

The addition of seeds to the synthesis gel (seeding process) makes itpossible however to eliminate this low-temperature maturing phase. Underthese conditions, it is thus possible to control the mean size of thecrystals by adjusting the amount of seeds introduced into the synthesisgel and thus form a reaction medium capable of forming zeolite crystals.

Therefore it is thus possible, or at the very least theoreticallypossible, to obtain zeolite crystals of various particle sizes, varyingfor example from several tens, or even several hundreds of nanometres toseveral tens of micrometres, it being understood that a synthesisreaction, with operating conditions suitable for this synthesis, resultsin the formation of zeolite crystals of relatively well-controlled,generally monomodal particle size characterized by a broader or narrowerparticle size distribution.

However, the fields of use of zeolites are today increasingly varied andincreasingly require elaborate technologies, so that it is oftennecessary to have available zeolites having a monomodal, bimodal or evenmultimodal controlled particle size, the full width at half maximum(FWHM) of which can be regulated and controlled.

Indeed, it may be necessary these days to be able to provide zeolitecrystals of controlled particle size, and more particularly of modulatedparticle size, in order for example to increase the compactness, thedensity, etc., depending on the targeted applications. In theseapplications, it is moreover often required to replace used zeolites,and it is therefore essential to be able to replace these used zeoliteswith new zeolites having the same particle size characteristics.

Versatile synthesis processes are therefore sought that make it possibleto obtain zeolites having bimodal or multimodal particle sizedistributions that are well controlled and above all modulated, that isto say repeatable over time.

Synthesis processes are known that result in zeolite crystals having arelatively narrow monomodal particle size distribution. However, besidesthe fact that it is often difficult to reproduce several identicalsuccessive syntheses (often “batch” syntheses) very accurately in orderto achieve the same particle size characteristics, these techniques donot generally make it possible to obtain multimodal particle sizedistributions.

In order to obtain zeolite crystals of different and well-defined sizes,it could be envisaged to produce physical mixtures of zeolite crystalshaving a perfectly well-defined monomodal distribution. Mixtures ofzeolite crystals, that is to say physical mixtures of dry powders, arein reality rather unsatisfactory; indeed, it is very difficult to obtainhomogeneous mixtures of crystals having particle sizes between severaltens of nanometres and several tens of micrometres.

Therefore, there remains a need for a process for preparing zeolitecrystals of controlled particle size, of modulated particle size, ofmonomodal or multimodal particle size distribution, with an adjustableFWHM, and having a particle size between several tens of nanometres andseveral tens of micrometres.

The Applicant has now discovered that the objectives described above canbe achieved in full or at the very least in part by means of the processdescribed below, which forms a first subject matter of the presentinvention. Other advantages and yet other subjects will appear in thedescription of the present invention that follows.

SUMMARY

The Applicant has now discovered that reproducible and homogeneous(stable throughout the production) multimodal particle sizedistributions of zeolite crystals can be easily obtained by mixing thereaction media from several synthesis reactors, at the end of or duringthe crystallization, which produce different crystal sizes, and byadjusting the proportions of the reaction media introduced.

In order to ensure good management of the multimodality of the particlesize distribution of zeolite crystals, the process of the presentinvention comprises:

-   -   the production in parallel, in at least two different synthesis        reactors, of zeolite crystals of different particle size        distributions, then    -   the mixing, after the start of the crystallization reaction,        that is to say during the crystallization reaction or at the end        of the crystallization, of the reaction media (suspensions of        zeolite crystals thus produced) of the at least two different        reactors, in proportions resulting in the desired multimodal        particle size distribution.

DETAILED DESCRIPTION

Thus, the present invention firstly relates to a process for preparingzeolite crystals having a multimodal particle size distribution, and thesizes of which are between 0.02 μm and 20 μm, said process comprising atleast the following steps:

-   a) preparing a synthesis gel by mixing at least one source of    silica, at least one source of alumina and optionally, but    preferably, at least one aqueous alkali metal or alkaline-earth    metal hydroxide solution,-   b) feeding at least two reactors each with a synthesis gel capable    of forming zeolite crystals,-   c) carrying out a crystallization reaction, in parallel, in each of    the at least two reactors,-   d) mixing the reaction media of the at least two reactors, and-   e) filtering the mixture of the reaction media obtained in step d),    in order to separate the crystals produced from the mother liquors.

The synthesis gel may be any type of composition well known to a personskilled in the art depending on the type of zeolite to be prepared andtypically comprises at least one source of silica and at least onesource of alumina, and/or any other source of element(s) that mayconstitute a zeolite framework, for example a source of phosphorus, oftitanium, of zirconium, etc. It is also possible, or even preferable, toadd at least one aqueous solution of an alkali or alkaline-earth metalhydroxide, preferably of an alkali metal hydroxide, typically of sodiumhydroxide and/or also organic structure-directing agents or templates.

A source of silica is understood to mean any source well known to aperson skilled in the art and in particular a solution, preferably anaqueous solution, of silicate, in particular of alkali or alkaline-earthmetal silicate, for example of sodium silicate, or of colloidal silica.

A source of alumina is understood to mean any source of alumina wellknown to a person skilled in the art and in particular a solution,preferably an aqueous solution, of aluminate, in particular of alkali oralkaline-earth metal aluminate, for example of sodium aluminate.

The concentrations of the various solutions of silica and alumina areadapted depending on the nature of the source of silica, of the sourceof alumina, the respective proportions of the sources of alumina and ofsilica to which the alkali or alkaline-earth metal hydroxide solutionand/or one or more organic structure-directing agents are added,according to the knowledge of a person skilled in the art. Informationwill in particular be found on the chemical nature of the organicstructure-directing agents to optionally be used as a function of thezeolite to be synthesized on the website of the International ZeoliteAssociation (www.iza-online.org), for example and non-limitinglytetramethylammonium (TMA), tetra-n-propylammonium (TPA),methyltriethylammonium (MTEA).

The respective proportions and concentrations of the various silica andalumina solutions are known to a person skilled in the art or may beeasily adapted by a person skilled in the art depending on the nature ofthe zeolite that it is desired to prepare, from data in the literature.

The synthesis gel from step a) is prepared as described above by mixingsources of silica and alumina in a basic medium. This mixing isadvantageously carried out in a rotor-stator shear mixer, that is to saya shear mixer comprising a rotor that rotates at high speed and thatmakes the mixture pass through a stator, the geometry of which may vary.

The degree of shear is defined by the shear rate γ in s⁻¹ which is equalto the tip speed of the rotor divided by the thickness of the gapbetween the rotor and the stator. The tip speed V_(p) is calculated fromthe speed of rotation V_(r) and from the diameter of the rotor daccording to the equation: V_(p)=V_(r)πd_(r) (expressed in m·s⁻¹), whereV_(r) is the speed of rotation expressed in rev·s⁻¹, d_(r) is thediameter of the rotor (expressed in m) and γ is equal to V_(p)/e, wheree represents the distance of the gap between the rotor and the stator(expressed in m).

The shear rate generally applied is between 10 000 s⁻¹ and 200 000 s⁻¹,preferably between 10 000 s⁻¹ and 100 000 s⁻¹.

The at least two reactors are each fed in step b) with a synthesis gelof zeolite crystals by any suitable means for transferring a fluid, forexample by gravity flow, by siphoning or by pumping. The monitoring ofthe flow rates of the synthesis gel at the inlet of each of the at leasttwo reactors and/or of the production of crystals at the outlet of eachof the at least two reactors may be obtained according to any meansknown to a person skilled in the art and preferably by means of pumps,optionally combined with flow regulators.

The at least two reactors are each fed by a synthesis gel of zeolitecrystals. The synthesis gels may be identical or different, that is tosay that they may be prepared from various silica and alumina solutionsintroduced in different respective proportions and concentrations thatare known to a person skilled in the art or that can be easily adaptedby a person skilled in the art depending on the nature of the zeolitethat it is desired to prepare, from data in the literature.

One preferred embodiment of the process of the present inventioncomprises the introduction of one or more seeding agents into thesynthesis gel(s) upstream of or inside at least one of the synthesisreactors or in the at least two synthesis reactors. A seeding agent isunderstood to mean a solution or a suspension, in liquid form or in theform of a gel, a solid or a liquid that promotes the orientation of thesynthesis towards the desired zeolite. Such solids and liquids thatpromote the orientation of the synthesis towards the desired zeolite arewell known to a person skilled in the art and are for example chosenfrom nucleating gels, zeolite crystals, mineral particles of any nature,etc., and also mixtures thereof.

According to a preferred aspect, the seeding agent is a nucleating gel,and, more preferably, said nucleating gel comprises a homogeneousmixture of a source of silica (for example sodium silicate), a source ofalumina (for example alumina trihydrate), optionally but advantageouslya strong mineral base, for instance sodium hydroxide, potassiumhydroxide or calcium hydroxide to mention but the main ones and the onesmost commonly used, and water. One or more structure-directing agents,typically organic structure-directing agents, may also optionally beintroduced into the nucleating gel.

The seeding agent(s) may be mixed with the synthesis gel according toany technique well known to a person skilled in the art and preferablyusing a static mixer, which has the advantage of promoting thehomogenization of said mixture.

The reactors used may be of any type well known to a person skilled inthe art and suitable for the type of synthesis envisaged, for examplestirred reactor for batch-mode syntheses and tubular reactor forcontinuous-mode syntheses. In the process of the invention generally 2or more reactors, of identical or different types, preferably 2, 3, 4 or5 reactors of identical or different types, more preferably 2 or 3reactors of identical or different types, more preferably 2 reactors ofidentical or different types, typically two reactors of identical types,are present.

It is moreover preferred to conduct the process continuously, and inthis case use will preferably be made of 2, 3 or 4 tubular reactors,more preferably 2 or 3 tubular reactors, typically 2 tubular reactors.

A “stirred reactor” is understood to mean a reactor equipped with astirring system, typically equipped with one or more agitators mountedon the same shaft or on different shafts, for example andnon-limitingly, with blade agitator(s), blender(s), or else anArchimedes screw type mixer and optionally equipped with one or moresystems of baffles or deflectors.

A “tubular reactor” is understood to mean a reactor or system ofreactors having length-to-diameter (or equivalent diameter) ratios ofgreater than 3, preferably of greater than 10 and more preferablygreater than 50, and defining a crystallization reaction zone subjectedat least in part to stirring means, whether these are stirring spindles,passive systems such as baffles, restrictions, rings or deflectors or anoscillating or pulsating system (enabling a back-and-forth movement ofthe reaction medium to be generated by means for example of a piston,membrane), etc., and also two or more of these techniques combined.

In a preferred embodiment of the invention, the tubular reactor isprovided with restrictions and equipped with a system that enablespulses to be imparted to the fluid circulating in the reactor, as forexample described in application US 2009/0304890 by NiTech.

Step c) of carrying out the crystallization reactions is conductedaccording to methods known to a person skilled in the art, that is tosay either by maturing/crystallization starting from a synthesis gel, orby direct crystallization after seeding by various types of seeds. Theseeded process is preferred in that it enables a better management ofthe size of the crystals produced.

The crystallization reactions are carried out in parallel, each in areactor, and may be carried out simultaneously and/or sequentiallyand/or successively, preferably simultaneously.

The crystallization reaction is generally carried out at hightemperature, that is to say at a temperature between 60° C. and 200° C.,preferably between 80° C. and 160° C. The crystallization of thesynthesis gel usually occurs spontaneously in the reactor and ispromoted by the seeding agent(s) added to said synthesis gel. Thecrystallization is also promoted by the temperature applied to thesynthesis gel, but also by any static or dynamic means of agitating thesynthesis gel within the reactor as explained above.

The term “promoted” is understood to mean a better initiation ofcrystallization and/or greater crystallization kinetics.

The crystallization reaction may be carried out under pressure, forexample under autogenous pressure, under atmospheric pressure, or moregenerally under any pressure, typically between atmospheric pressure and1.5 MPa.

The reaction media of the at least two reactors operating in parallelare then mixed at any moment, as soon as the crystallization reactionhas begun in one of the at least two reactors, preferably in the atleast two reactors, this being in the proportion that enables thedesired multimodal distribution to be obtained.

The crystallization reaction is considered to have begun in the reactoras soon as the degree of crystallinity, analyzed by x-ray diffraction(XRD) on a sample withdrawn from said reactor then dried at 80° C. for 4hours, is greater than 5%, preferably greater than 10%, more preferablygreater than 30% and very preferably greater than 50%.

When the syntheses are carried out in continuous mode, for each of thereactors, which are preferably tubular reactors, the synthesis gel ispreferably prepared continuously using a shear mixer operatingcontinuously. It is then possible to introduce into the synthesis gel,by any suitable means described above and continuously, one or moreseeding agents in order to seed and adjust the size of the crystals thatwill be obtained after the crystallization.

It may also be envisaged to operate with seeding agents and/or synthesisgels of different natures for the purpose of obtaining zeolite crystalsthat are themselves of different structures. The process of the presentinvention may specifically be implemented in order to synthesize zeolitecrystals of different structures.

Several reactors (at least two) operate in parallel, at suitable flowrates, in order to adjust the proportion of the various size categoriesof the zeolite crystals. The reaction media produced continuously inthese reactors and that contain the zeolite crystals are mixedcontinuously as soon as the crystallization reaction has begun in one ofthe at least two reactors, preferably in the at least two reactors, thisbeing in the proportion that enables the desired multimodal distributionto be obtained.

According to a preferred embodiment, the process of the presentinvention is carried out in continuous mode. Unlike batch mode,continuous mode offers the advantage of facilitating the adjustment ofthe multimodal distribution by controlling the flow rates of eachtubular reactor.

The process of the present invention is characterized by the fact thatat least two syntheses of zeolite crystals are carried out in parallel,the reaction media of which are combined as soon as the crystallizationreaction has begun in one of the at least two reactors, preferably inthe at least two reactors, this being in the proportion that enables thedesired multimodal distribution to be obtained.

The mixing of the reaction media may be carried out according to anymethod well known to a person skilled in the art, the methods that areparticularly suitable are those that enable an effective and homogeneousmixing of aqueous media in which solid particles are in suspension.Among the methods that can be used, mention may be made, as nonlimitingexamples, of those using a static mixer, or any other type of blademixer, propeller mixer, multiple paddle mixer, or else a simple systemof pipes that join up into a single pipe (for example “Y” tube in thecase of two reactors). It is also possible to combine one or more mixingtechniques.

The fact of mixing reaction media containing the zeolite crystals in theform of liquid suspensions makes it possible to obtain, afterfiltration/washing and drying, a mixture of zeolite crystals that ishomogeneous in terms of particle size distribution. This result cannotbe obtained easily, that is to say economically and with goodreproducibility, when dried zeolite crystals of different particle sizesare mixed. Specifically, in this case, segregation phenomena are oftenobserved that are inherent to the dry mixing of zeolite crystals ofvarious sizes, therefore of various individual masses.

The reaction media resulting from the crystallization step c) aredifferent in that they contain zeolite crystals that are different,either in terms of size, or in terms of nature, or in terms of natureand size. It is however preferred to use the process of the inventionfor the synthesis of a single crystalline form of zeolite, with howeverdifferent particle size distributions, so as to obtain zeolite crystalshaving a bimodal or multimodal particle size distribution.

A person skilled in the art easily understands the great flexibilityimparted by the process of the invention, enabling populations ofzeolite crystals having controlled and modulated particle sizedistributions to be generated.

For example, the respective flow rates and/or the respective amounts ofeach of the reaction media may be adjusted in order to regulate theproportions of each of the reaction media entering the mixing step d)and thus to easily control the desired particle size distribution of themixture of zeolite crystals at the end of the process of the presentinvention.

The process of the invention thus makes it possible to obtain amultimodal size distribution of crystals that is adjustable andmodulated by mixing several reaction media, each of them being derivedfrom a synthesis, the parameters of which impose a well determined andmodulated particle size.

By means of the process of the invention, it is now possible to preparezeolite crystals industrially, advantageously continuously, by finelyadjusting the number-size distribution of the zeolite crystals. It isthus possible to obtain an easy, effective and economical industrialproduction of zeolite crystals with a multimodal particle sizedistribution that is adjustable and modulated, that is to sayreproducible and stable over time.

In one preferred embodiment, the process of the invention is a processfor the continuous synthesis of a mixture of zeolite crystals having abimodal particle size distribution, said process being carried out intwo tubular reactors operating in parallel with different synthesisconditions so as to produce crystals of different particle size, theoutlets of the two reactors being joined together by means of Y-shapedtubing.

According to a preferred aspect of the present invention, the reactionsfor the syntheses of the zeolite crystals are carried out in thepresence of one or more seeding agents, as defined above.

At the end of the mixing step d), the mixture of said reaction media isfiltered in order to separate the crystals produced on the one hand andthe mother liquors on the other hand. This filtration may be carried outaccording to any method well known to a person skilled in the art, andfor example by one or more methods chosen from centrifugation, filterpress filtration, belt filter filtration, rotary filter filtration, etc.

The crystals obtained at the end of step e) may optionally be subjectedto one or more conventional treatments well known to a person skilled inthe art, such as washing, cation exchange, drying, impregnation,activation, etc., it being possible for this or these treatment(s) to becarried out in batch mode or continuously, advantageously continuously.For example, the crystals obtained may be subjected to one or morewashings with water, so as to eliminate the residual mother liquors thatmight still be present.

The crystals obtained may also be dried, according to the conventionaltechniques for drying zeolite crystals, for example at temperaturesbetween 40° C. and 150° C., for a duration that may vary between severalminutes and several hours, typically between several minutes and 10hours. The drying operation at a temperature below 40° C. could prove tobe much longer and thus economically unprofitable, whereas a dryingtemperature above 150° C. could result in a greater or lesserdeterioration of the still wet zeolite crystals.

After drying, the zeolite crystals may be used as is, but they areadvantageously activated, here too according to conventional activationtechniques well known to a person skilled in the art, for example attemperatures between 150° C. and 800° C., for a duration that variesfrom several minutes to several hours, and typically from severalminutes to 10 hours.

The mother liquors resulting from the filtration step e) mayadvantageously be recycled. One of the advantages of this recycling isto thus enable the reduction in the consumption of sodium hydroxide byintroducing the mother liquors directly into the reaction medium of oneof the at least two reactors or into the silicate solution or else intothe aluminate solution (typically that are respectively the sources ofsilica and of alumina in step a) of the process), or else into thesynthesis gel, but may also enable a substantial reduction in energyconsumption. Before being recycled, the mother liquors may optionallyhave undergone one or more treatments chosen from ultrafiltration,reconcentration, distillation, etc.

The process of the present invention is very advantageously carried outcontinuously, preferably completely continuously, that is to say withouta step in batch mode.

Thus, according to one embodiment of the process of the presentinvention, each reactor delivers a given and advantageously narrowparticle size fraction determined by the quality and the amount ofseeding agent(s) introduced into the synthesis gel. The total amount ofseeding agent(s) added to the process of the present inventionrepresents between 0.005% and 10% by weight relative to the synthesisgel, preferably between 0.01% and 5% and more preferably between 0.01%and 3% by weight relative to the synthesis gel introduced at the startinto each reactor. Subsequently, the various reaction media are mixed toobtain a multimodal size distribution which is the sum of the variousmonomodal fractions produced in each of the synthesis reactors.

Thus, the process of the present invention enables the synthesis,advantageously continuously, of zeolite crystals having a multimodalparticle size distribution, this being in a manner that is homogeneousand reproducible, and stable over time.

The determination of the particle size distribution corresponds here tothe particle number-size distribution of the diameter of the zeolitecrystals. This determination is carried out from images obtained byobservation under a scanning electron microscope (SEM). For this, a setof images is taken at a magnification of at least 3000. All of thecrystals present on the images are measured using dedicated software,for example the Smile View software published by LoGraMi, so as tomeasure at least 300 crystals, then the distribution by number isplotted in the form of a histogram with categories adapted to theparticle size of the crystals, for example categories every 0.2 μm forcounting micrometric crystals or for example categories every 0.02 μmfor counting crystals of several tens of nanometres.

A “multimodal particle size distribution” is understood to mean adistribution of multimodal size, that is to say having at least two“separate” peaks, in other words at least two “resolved” peaks. Thevalue of the diameter at the top of the peak is referred to as “mode” orelse “dominant value”, and represents the most common value of the peak.When a distribution has two separate (or resolved) peaks, thedistribution is said to be bimodal.

The concept of multimodality is defined using a “criterion” R, known asthe “resolution factor” which characterizes the separation ornon-superimposition of the peaks.

The various peaks are compared to a Gaussian, characterized by its moded and its full width at half maximum 6, from which it is possible todeduce the width of the base of the peak ω=1.7 δ.

The resolution factor R of 2 adjacent peaks A and B is conventionallydefined (see for example: “Notions fondamentales de chromatographie”[Fundamental chromatography concepts] by Marie-Paule Bassez:http://chemphys.u-strasbg.frimpb/teach/chromato1/img0.html) using thefollowing equation:R=2(dB−dA)/(ωB+ωA),where dA and dB are respectively the modes of the peaks A and B (in μm),and ωA and ωB are respectively the widths of the base of the peaks A andB (in μm).

As a general rule, two peaks are considered to be resolved, or elsecompletely separate, when the value of R is greater than 1.5. In thecontext of the present invention, a particle size distribution has adifference in modality when the resolution factor R is greater than 0.5.In the present description, the particle size distribution is consideredto be multimodal since at least two peaks are resolved. When theparticle size distribution comprises only two resolved peaks, referenceis then made to a bimodal particle size distribution.

The process according to the present invention thus enables theproduction of zeolites, the crystals of which have a controlled or evenmodulated, bimodal or even multimodal, particle size distribution, itbeing possible for this production to be carried out industrially veryeasily, thus enabling a production of large amounts of such zeolites ofcontrolled or even modulated particle size, with production costs muchlower than those observed, for example, with productions according tothe conventional processes known today.

The zeolites that can be prepared according to the process of thepresent invention may be of any type and, for example andnon-limitingly, any zeolite of MFI type, and in particular silicalite,any zeolite of MOR type, of OFF type, of MAZ type, of CHA type and ofHEU type, any zeolite of FAU type, and in particular zeolite Y, zeoliteX, zeolite MSX, zeolite LSX, any zeolite of EMT type or else any zeoliteof LTA type, i.e. zeolite A, and also the other zeotypes, such as forexample titanosilicalites.

The term “zeolite MSX” (medium silica X) means a zeolite of FAU typewith an Si/Al atomic ratio of between about 1.05 and about 1.15, limitsincluded. The term “zeolite LSX” (low silica X) means a zeolite of FAUtype with an Si/Al atomic ratio equal to about 1.

The process according to the invention is particularly suitable for thepreparation of zeolites chosen from zeolites of MFI type, and inparticular silicalite, of FAU type, and in particular zeolite Y, zeoliteX, zeolite MSX, zeolite LSX, and of LTA type, i.e. zeolite A, and alsothe zeolites of CHA type and the zeolites of HEU type.

The process according to the invention is moreover very particularlysuitable for the preparation of any zeolite of FAU type, and inparticular zeolite X, zeolite MSX, zeolite LSX. The zeolites of MFItype, and in particular silicalite, may also be very advantageouslyprepared according to the process of the invention.

Moreover, the continuous preparation process of the present invention isnot limited to the preparation of the zeolites described above, but alsoincludes the corresponding zeolites with hierarchical porosity. Thezeolites with hierarchical porosity are solids comprising a microporousnetwork linked to a network of mesopores, and thus make it possible toreconcile the properties of accessibility to the active sites of themesoporous zeolites known from the prior art and those of maximumcrystallinity and maximum microporosity of “conventional” zeolites(without mesoporosity). In this case, specific structure-directingagents are introduced into the reaction medium of step a), for examplestructure-directing agents of organosilane type as described in documentFR 1 357 762.

The synthesis process of the present invention consequently enables aneasy and economical industrial synthesis of zeolite crystals, the atleast bimodal particle size distribution of which is homogeneous,controlled, or even modulated. Moreover, it has been observed that theprocess according to the invention is very stable over time when it iscarried out in continuous mode. These zeolite crystals find veryinteresting uses in many fields of application.

Specifically, owing to the process of the invention, it is now possibleto more easily obtain zeolite crystals of multimodal distribution in amodulated and homogeneous manner, unlike what would be obtained withmixtures of crystals of various particle sizes.

The multimodal particle size distribution of the zeolite crystalsobtained by means of the process of the invention makes it possible toobtain crystals having especially a bulk density that is high, and inparticular higher compared to crystals of monomodal particle sizedistribution. Specifically, the small crystals can be considered tooccupy the spaces between the large crystals.

This high bulk density of the zeolite crystals obtained with the processof the invention makes it possible to obtain a very distinctiveadsorption performance, especially in terms of adsorption volumecapacity.

The zeolite crystals obtained with the process of the invention thusfind very interesting applications in the field of adsorption,separation and purification of gases and liquids. As nonlimitingexamples, the zeolite crystals obtained according to the process of thepresent invention may advantageously be used as adsorbent fillers inpolymer-based composites, as a constituent of agglomerated zeoliteadsorbents used in adsorption separation or purification processes suchas pressure swing and/or temperature swing processes, or else inchromatographic type separation processes (fixed beds, moving beds,simulated moving beds), in applications as varied as the purification ofindustrial gases, the separation of nitrogen and oxygen, thepurification of natural gas or of synthesis gas, or else thepurification of various petrochemical fractions, the separation ofisomers in refining, etc.

The degree of crystallinity, and also the purity of the zeolitesynthesized, are evaluated by x-ray diffraction analysis, a techniqueknown to a person skilled in the art under the acronym XRD. Thisidentification is for example carried out on a Bruker XRD apparatus.

This analysis makes it possible not only to determine the amount ofcrystalline phase(s) present, but also to identify and quantify thepossible different zeolites present, each of the zeolites having aunique diffractogram defined by the positioning of the diffraction peaksand by the relative intensities thereof. The noncrystalline phases arenot detected by the x-ray diffraction analysis.

This analysis is also used to determine the degree of crystallinity ofthe reaction medium in order to evaluate whether the crystallizationreaction has begun. In this case, a sample of the reaction medium iswithdrawn, dried at 80° C. for 4 hours, then analysed by XRD.

The zeolite crystals (or the dried samples) are ground and then spreadand levelled out on a sample holder by simple mechanical compression.The conditions under which the diffractogram is acquired on the BrukerD5000 machine are as follows:

-   Cu tube used at 40 kV-30 mA;-   slit size (divergent, scattering and analysis slits)=0.6 mm;-   filter: Ni;-   sample device rotating at: 15 rpm;-   measuring range: 3°<2θ<50°;-   increment: 0.02°;-   counting time per increment: 2 seconds.

Interpretation of the diffractogram obtained is performed with the EVAsoftware with identification of the zeolites using the ICDD PDF-2release 2011 database.

The amount of crystals, by weight, is determined by XRD analysis; thismethod is also used to measure the amount of noncrystalline phases. Thisanalysis is performed on a Bruker machine, and the amount by weight ofthe zeolite crystals is then evaluated using the TOPAS software from thecompany Bruker.

The crystallinity (or degree of crystallinity) corresponds to the ratioof the sum of the weight fractions of the crystalline phases present,relative to the total weight of the sample.

The purity is expressed as a weight percentage of desired crystallinephase relative to the total weight of the sample.

The invention claimed is:
 1. Process for preparing zeolite crystalshaving a multimodal particle size distribution, where the particle sizesare between 0.02 μm and 20 μm, said process comprising at least thefollowing steps: a) preparing a synthesis gel by mixing at an appliedshear rate of between 10,000 s⁻¹ and 200,000 s⁻¹ at least one source ofsilica, at least one source of alumina and optionally, at least oneaqueous alkali or alkaline-earth metal hydroxide solution, where thesynthesis gel contains between 0.005% and 10% by weight of seedingagent(s) relative to the synthesis gel, b) feeding each of at least tworeactors directly with the synthesis gel obtained in step a), where thesynthesis gel is capable of forming zeolite crystals and is identical ordifferent between the reactors, c) carrying out a crystallizationreaction, in parallel, in each of the at least two reactors, where thereactions are carried out simultaneously, sequentially or successively,to form zeolite crystals of different particle size distributions, d)mixing the reaction media from the crystallization reactions of the atleast two reactors at a time after start of the crystallizationreaction, and e) filtering the mixture of the reaction media obtained instep d), in order to separate the crystals produced from the motherliquors, wherein one or more seeding agents are introduced into thesynthesis gel(s) upstream of or inside at least one of the at least tworeactors.
 2. Process according to claim 1, wherein the seeding agent ischosen from nucleating gels, zeolite crystals, mineral particles, andmixtures thereof.
 3. Process according to claim 1, wherein the reactorsare selected from stirred reactors for syntheses in batch mode andtubular reactors for syntheses in continuous mode.
 4. Process accordingto claim 3, wherein 2, 3 or 4 of the reactors are tubular reactors. 5.Process according to claim 1, wherein two or more of the reactors areeither stirred reactors or tubular reactors.
 6. Process according toclaim 1, wherein the crystallization reaction is carried out in the atleast two reactors at a temperature of between 60° C. and 200° C. 7.Process according to claim 1, wherein the crystallization reaction iscarried out in the at least two reactors under a pressure betweenatmospheric pressure and 1.5 MPa.
 8. Process according to claim 1,wherein the reaction media of the at least two reactors operating inparallel are mixed at a time after start of the crystallization reactionbut before the time in mixing step d).
 9. Process according to claim 1,wherein the mother liquors from the filtration step e) are recycled. 10.Process according to claim 1, wherein the zeolite crystals are crystalsof zeolite selected from the group consisting of zeolites of MFI type,zeolites of MOR type, zeolites of OFF type, zeolites of MAZ type,zeolites of CHA type, zeolites of HEU type, zeolites of FAU type,zeolites of EMT type and zeolites of LTA type.